Abstract:

A method and device for the production of polymer filaments with a
diameter of less than one micron. A plurality of polymer components are
extruded through a spin pack and then attenuated using gas flows which
are accelerated to achieve high velocity by means of a converging,
diverging nozzle. The plurality polymer components may be extruded in an
islands in the sea or segmented pie configuration. As a result of the
high velocity gas flow, the plural components are split apart into their
individual components resulting in filaments and fibers having a diameter
or minor dimension of less than one micron.

Claims:

1. A method for producing submicron fibers by melt spinning a plurality of
polymers through a spinneret die in a plural component configuration to
produce a multiconstituent fiber, and splitting the plural components
into their individual constituents by a high velocity gas stream, said
high velocity gas stream being applied to the fiber while it exits the
die in a way that multiple fibers are formed from at least one of the
individual constituents.

2. The method of claim 1 in which the gas stream has a velocity of between
0.7 and 1.4 times the speed of sound.

3. The method of claim 1 in which the plural component configuration is
"islands in the sea" or "segmented pie".

4. The method of claim 1 further comprising the step of applying an
electrostatic charge to the filaments.

5. An apparatus comprising a spin pack with distribution channels and
orifices arranged such that a plurality of polymers can be coextruded in
a plural component configuration, and a converging diverging gas nozzle.

7. A method for separating the individual components of multiple component
fibers comprising co-extruding the filaments through a spinneret die and
splitting the plural components into their individual parts by a high
velocity gas nozzle prior to complete solidification of the components.

Description:

FIELD OF THE INVENTION

[0001]This invention relates to extrusion of polymer fibers, and in
particular fibers that have a diameter of less than one micron.

BACKGROUND OF THE INVENTION

[0002]Fibers and filaments have been produced for many years using methods
well known as melt blowing and spun laid fiber spinning. Melt blowing is
typically associated with fibers of finite length whereas the spun laid
process is typically associated with continuous filaments. In both of
these technologies there has been an extended effort from many
individuals to reduce the diameter of the fibers produced. Typical
minimum fiber and filament diameters for these technologies is now on the
order of 3 to 5 microns. One method for producing finer fibers is known
as bicomponent spinning. In this method, disclosed for example in U.S.
Pat. No. 5,162,074 to Hills and incorporated herein by reference, two or
more polymers are extruded through specially designed spin packs which
configure the filaments into arrangements known as side by side,
sheath/core, islands in the sea or segmented pies. Of these arrangements,
those of the islands in the sea and segmented pies are such that although
the filaments of the combined components exceed 1 micron in diameter, the
individual components can be separated from each other by post processing
to result in filaments with a diameter of less than 1 micron. This post
processing typically includes mechanical action to fracture the
components apart at the segmented pie interfaces or chemically dissolving
the sea polymer to leave only the islands polymer.

[0003]These post processing steps can be both costly an inefficient. An
article "Spunbonded nonwovens made from splittable bicomponent filaments"
by Schilde, Erth, Heye and Blechschmidt, Chemical Fibers International
Vol 57 No 1, March 2007 describes multiple methods and the difficulties
encountered in mechanical splitting of these fibers. Islands in the sea
filaments provide the smallest known fiber diameters from melt polymers,
ref "Spinning of Submicron diameter Fibers" and "Production of Sub-Micron
Fibers in Nonwoven Fabrics" by Hagewood on Hills Inc website hillsinc.net
where as many as multiple thousand of island fibers can exist within a
single bicomponent filament. Complete removal of the sea polymer,
however, is a known issue with this technology as evidenced by art
disclosed to facilitate this process. See for example U.S. Pat. No.
6,861,142 "Controlling the dissolution of dissolvable polymer components
in plural component fibers".

[0004]As a result of these difficulties, recent developments to reduce the
fiber diameters have focused primarily on reduction of the size and
spacing of the spinneret holes as disclosed for example by Allen, US
Application US2005/0087900 for spunbonding and by Berger, U.S. Pat. No.
7,192,550 for melt blowing.

[0005]An alternative method for making fine fibers has recently been
introduced in U.S. Pat. No. 6,800,226 to Gerking which does not reduce
the size of the spinneret holes, but rather adds a high velocity gas
nozzle below a fairly conventional melt blowing spin pack. Gerking
discloses that the high velocity gas causes the single polymer filament
to spontaneously burst into a plurality of smaller filaments. Gerking,
however, can not consistently reach the small fiber sizes achieved by the
bicomponent methods.

[0006]Each of these methods have shortcomings. The bicomponent spun laid
method using islands in the sea or segmented pie fibers requires post
processing. The reduced spin hole size methods suffer from productivity
reductions. Gerking's fiber bursting method suffers from a broad fiber
size distribution with a significant amount of larger fibers. In
addition, both Gerking and the new small hole methods rely on reducing
the melt viscosity of the polymers which results in some loss of fiber
properties. Thus, there still exists a need to produce fine fibers from
melt polymers in a way that has high productivity and a narrow fiber size
distribution.

SUMMARY OF THE INVENTION

[0007]In a preferred embodiment of the process a plurality of polymers is
spun through a spin pack designed to produce islands in the sea or
segmented pie bicomponent filaments and then through a converging
diverging gas nozzle. The spin pack is designed such that the bicomponent
filaments are extruded through a single row of holes. The tip of the spin
pack is tapered to direct the gas flow toward the extruded filaments. The
gas nozzle is designed with a converging diverging cross section so that
the gas velocity may reach sonic or even supersonic velocity.

[0013]FIG. 6 shows a second example of a configuration of gas nozzles.

DETAILED DESCRIPTION OF THE INVENTION

[0014]By "plurality" is meant more than one.

[0015]By "plural component configuration" in the context of polymer
coextrusion is meant that a plurality of polymers form distinct extrudate
phases that are present along the cross section of the entire length of
the fiber. Each phase shares a boundary with at least one other phase and
the number of phases does not necessarily equal the number of polymers in
the plurality. In other words, some of the phases may be multicomponent.

[0016]The process of the invention is directed to a method for producing
submicron fibers by melt spinning a plurality of polymers through a
spinneret die in a plural component configuration and splitting the
plural components into their individual parts by a high velocity gas
nozzle.

[0017]Fiber from any melt processible polymers can be produced by this
method, such as polyesters, polyamides, polyolefins and many other
polymers, but it is preferable to choose polymers that will facilitate
the bursting of the filaments along the defined interfaces of the
components. For example, polyethylene terephthalate (PET) and
polyethylene (PE) may be spun with polyester as the island polymer and
polyethylene as the sea polymer. The choice of a high melt flow
polyethylene with a standard viscosity polyester will enhance the
bursting of the filaments along the weak boundaries without loss of
polyester fiber properties due to excessive heating or polymer
degradation.

[0018]When an island in the sea configuration is used then preferably the
island fibers comprise more than 50%, or more preferably, more than 75%
of the bicomponent filament, then the amount of sea polymer in the final
product is reduced. Once the filaments have been burst into their
individual components by the high velocity gas flow, there is no
difficulty in separating the sea polymer from the island polymer. The
bursting causes the island polymer to retain their shape as well defined
filaments while the sea polymer fragments into particles and fibers. If a
sea polymer is chosen such that it is an easily dissolvable polymer, such
as polyvinyl alcohol, the removal issues in conventional bicomponent
fibers are no longer present. As an alternative, the sea polymer can be
chosen to add functionality to the final product. For example, in the
combination cited above of PET/PE, the PE can be used as a bonding agent.
A post heat treatment or calendering operation will cause the PE
fragments and fibers to bond the PET filaments to create a strong
nonwoven sheet.

[0019]Whether islands in the sea or segmented pie configurations are used,
the final fiber size distribution in the nonwoven product will be more
precise than conventional melt blowing. The fibers produced by the
process of the invention do not have to be circular in cross section. It
should be noted that as the percentage of island polymer is increased
above about 50%, the island filaments tend toward hexagonal type packing
creating flat sided filaments as opposed to circular cross sections. In
addition, with segmented pie or hollow segmented pie filaments, the
individual components are wedge shaped. In any of these cases, the
diameter or minor dimensions of the individual components are controlled
such that they are less than 1 micron.

[0020]Another preferred embodiment comprises the above method plus
electrostatic charging of the filaments. Numerous methods for charging of
the filaments would be known to one skilled in the art, such as Kubik
U.S. Pat. No. 4,215,682; Deeds U.S. Pat. No. 5.122.048; and Moosmayer
U.S. Pat. No. 4,904,174 all incorporated herein by reference. Any of
these methods can be adapted to the current invention so that the
individual filaments, once burst, will remain independent and not
re-coalesce. The preferred method for inducing charge in the fibers is a
corona discharge. Without meaning to be limited by mechanism, the
addition of electrostatic charge induces a repulsive force between the
individual filaments thus improving the overall fiber letdown.

[0021]The design of the spinneret of the process can best be appreciated
with reference to the figures. In FIGS. 1-3 are shown examples of fiber
configurations for a two polymer system. FIG. 1 shows an "island in the
sea" configuration. FIG. 2 shows a "segmented pie" configuration, and
FIG. 3 shows a side by side configuration. All three of these
configurations can be used in the process of the invention, but the
invention is not limited to them, and any configuration in which a
plurality of phases coexist in the cross section of the fiber and along
the length of the fiber can be used.

[0022]In FIG. 4 a schematic diagram shows the various major components of
the apparatus. In the example shown in FIG. 4, two polymers are fed to
the apparatus via inlets 41 and 42. The invention is not limited to two
polymers and multiple inlets can be used.

[0023]The polymers then flow through a set of distribution plates (43)
that feed the polymers to a tapered die tip (44). The polymers entering
the tip are essentially in the desired configuration required before melt
splitting, for example in the configurations of FIGS. 1-3.

[0024]Gas is fed to the apparatus through an inlet (48) and into a nozzle
(45). The nozzle has the effect of accelerating the gas to a velocity in
the range of 0.7 to 1.4 times the speed of sound. Fiber and gas then exit
the apparatus together, and optionally past needles (46) to which an
electrostatic charge is applied. The needles (46) are mounted in an
electrostatic insulator plate (47) to prevent arcing to the bottom of the
spin pack.

[0025]The gas nozzles may be arranged in the bottom plate as a row of
individual circular nozzles corresponding to the polymer die holes on a
one to one basis as shown in FIG. 5. Alternatively, the gas nozzle may be
configured as a slot jet as shown in FIG. 6.

[0026]Although the invention has been described herein in a particular
configuration, it will be understood that one skilled in the art will be
able to make changes to the process and apparatus described here that
fall within the scope of the invention and the claims below.